Add the virDomainDeviceAddress information to the sound, video
and watchdog devices. This means all of them gain the new XML
element

  <address .... />

This brings them upto par with disk/net/hostdev devices which
already have address info

* src/conf/domain_conf.h: Add virDomainDeviceAddress to sound,
  video & watchdog device struts.
* src/conf/domain_conf.c: Hook up parsing/formatting for
  virDomainDeviceAddress in sound, video & watchdog devices
* docs/schemas/domain.rng: Associate device address info
  with sound, video & watchdog